System for managing refractory periods in a cardiac rhythm management device with biventricular sensing

ABSTRACT

A method and system for managing refractory periods in a cardiac rhythm management device configured for biventricular or biatrial sensing. Refractory periods for each channel of the pacemaker are provided by interval timers that are triggered by sensed or paced events in order to prevent misinterpretation of sensing signals.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.12/254,615, filed on Oct. 20, 2008, now issued as U.S. Pat. No.8,271,083, which is a continuation of U.S. patent application Ser. No.11/218,865, filed on Sep. 2, 2005, now issued as U.S. Pat. No.7,440,802, which is a continuation of U.S. patent application Ser. No.10/420,178, filed on Apr. 21, 2003, now issued as U.S. Pat. No.7,191,001, which is a continuation of U.S. patent application Ser. No.09/748,733, filed on Dec. 26, 2000, now issued as U.S. Pat. No.6,553,258, the specifications of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to methods and systems for operating a cardiacrhythm management device. In particular, the invention relates todefining and managing periods during which sensing channels of thedevice are rendered refractory.

BACKGROUND

Cardiac rhythm management devices are implantable devices that provideelectrical stimulation to selected chambers of the heart in order totreat disorders of cardiac rhythm. A pacemaker, for example, is acardiac rhythm management device that paces the heart with timed pacingpulses. The most common condition for which pacemakers are used is inthe treatment of bradycardia, where the ventricular rate is too slow.Atrio-ventricular conduction defects (i.e., AV block) that are fixed orintermittent and sick sinus syndrome represent the most common causes ofbradycardia for which permanent pacing may be indicated. If functioningproperly, the pacemaker makes up for the heart's inability to paceitself at an appropriate rhythm in order to meet metabolic demand byenforcing a minimum heart rate. Pacing therapy may also be applied inorder to treat cardiac rhythms that are too fast, termedanti-tachycardia pacing. (As the term is used herein, a pacemaker is anycardiac rhythm management device with a pacing functionality, regardlessof any other functions it may perform such as cardioversion ordefibrillation.)

Also included within the concept of cardiac rhythm is the degree towhich the heart chambers contract in a coordinated manner during acardiac cycle to result in the efficient pumping of blood. The heart hasspecialized conduction pathways in both the atria and the ventriclesthat enable the rapid conduction of excitation (i.e., depolarization)throughout the myocardium. These pathways conduct excitatory impulsesfrom the sino-atrial node to the atrial myocardium, to theatrio-ventricular node, and thence to the ventricular myocardium toresult in a coordinated contraction of both atria and both ventricles.This both synchronizes the contractions of the muscle fibers of eachchamber and synchronizes the contraction of each atrium or ventriclewith the contralateral atrium or ventricle. Without the synchronizationafforded by the normally functioning specialized conduction pathways,the heart's pumping efficiency is greatly diminished. Patients whoexhibit pathology of these conduction pathways, such as bundle branchblocks, can thus suffer compromised cardiac output.

Patients with conventional pacemakers can also have compromised cardiacoutput because artificial pacing with an electrode fixed into an area ofthe myocardium does not take advantage of the above-describedspecialized conduction system. This is because the specializedconduction system can only be entered by impulses emanating from thesino-atrial or atrio-ventricular nodes. The spread of excitation from asingle pacing site must proceed only via the much slower conductingmuscle fibers of either the atria or the ventricles, resulting in thepart of the myocardium stimulated by the pacing electrode contractingwell before parts of the chamber located more distally to the electrode,including the myocardium of the chamber contralateral to the pacingsite. Although the pumping efficiency of the heart is somewhat reducedfrom the optimum, most patients can still maintain more than adequatecardiac output with artificial pacing.

Heart failure is clinical syndrome in which an abnormality of cardiacfunction causes cardiac output to fall below a level adequate to meetthe metabolic demand of peripheral tissues and is usually referred to ascongestive heart failure (CHF) due to the accompanying venous andpulmonary congestion. CHF can be due to a variety of etiologies withischemic heart disease being the most common. Some CHF patients sufferfrom some degree of AV block or are chronotropically deficient such thattheir cardiac output can be improved with conventional bradycardiapacing. Such pacing, however, may result in some degree ofuncoordination in atrial and/or ventricular contractions due to the wayin which pacing excitation is spread throughout the myocardium asdescribed above. The resulting diminishment in cardiac output may besignificant in a CHF patient whose cardiac output is alreadycompromised. Intraventricular and/or interventricular conduction defects(e.g., bundle branch blocks) are also commonly found in CHF patients. Inorder to treat these problems, cardiac rhythm management devices havebeen developed which provide pacing stimulation to one or more heartchambers in an attempt to improve the coordination of atrial and/orventricular contractions, termed cardiac resynchronization therapy.

In conventional pacemakers with sensing channels for sensing one or moreheart chambers, the ventricular and/or atrial sensing channels arerendered refractory following certain events, such that certain sensedevents are ignored for the duration of the period. Sensing channels arerendered refractory both in order to prevent reentry into the system ofan output pacing pulse (in which case the sensing amplifiers areblanked) and to prevent the misinterpretation of input data by thesensing of afterpotentials or by crosstalk between sensing channels.Cardiac resynchronization therapy may involve pacing both atria, bothventricles, or a heart chamber at one or more pacing sites based uponsenses from another site. In order to control the pacing and avoidpacing a chamber or site in the presence of intrinsic activity, sensingchannels should be provided for each chamber or site. Sensing bothventricles or both atria, however, requires that refractory periods bemanaged differently from the situation where only one atria or oneventricle is sensed.

SUMMARY OF THE INVENTION

The present invention is a system and method for managing the refractoryperiods of sensing channels in a cardiac rhythm management device inwhich both ventricles, both atria, and/or multiple sites in the sameheart chamber are sensed. Such sensing configurations can most usefullybe employed in delivering cardiac resychronization therapy. Therefractory periods for the sensing channels are managed in a manner suchthat misinterpretation of sense signals is avoided while still allowingthe device to pace one or more chambers safely and at the appropriatetimes.

In an exemplary cardiac resynchronization pacing configuration, heartchambers designated as a rate chamber and a synchronized chamber aresensed through separate channels, and at least one chamber is paced uponexpiration of an escape interval without receipt of a rate chamber sensesignal. In accordance with the invention, each sensing channel isrendered refractory for a separately selected pacing refractory periodafter a pacing event, and a sensing channel is rendered refractory for aselected sensing refractory period after receipt of a sense signal fromthat channel while leaving the other channel non-refractory.

In a particular embodiment of the invention, the rate and synchronizedchambers are the right and left ventricles, respectively, and an atrialsensing and pacing channel is also present. The atrial sensing channelmay be rendered refractory for selected intervals following an atrialsense, an atrial pace, a pace of either ventricle, or a rightventricular sense signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of a pacemaker configured for biventricularpacing and sensing.

FIG. 2 shows refractory periods following a sensed atrial event.

FIG. 3 shows refractory periods following an atrial pace.

FIG. 4 shows refractory periods after a right ventricular sense.

FIG. 5 shows refractory periods after a left ventricular sense.

FIGS. 6 and 12 show refractory periods following a ventricular pace inbiventricular pacing mode with a positive biventricular delay.

FIG. 7 shows refractory periods following a ventricular pace inbiventricular pacing mode with a positive biventricular delay where theleft ventricular pace is inhibited.

FIGS. 8 and 11 show refractory periods following a ventricular pace inbiventricular pacing mode with a negative biventricular delay.

FIG. 9 shows refractory periods following a ventricular pace inbiventricular pacing mode with a negative biventricular delay where theleft ventricular pace is inhibited.

FIG. 10 shows refractory periods following a ventricular pace inbiventricular pacing mode with simultaneous ventricular pacing.

FIG. 13 shows refractory periods following a ventricular pace in a rightventricle-only pacing mode.

FIG. 14 shows refractory periods following a ventricular pace in a leftventricle-only pacing mode.

FIG. 15 shows refractory periods following a right ventricular safetypace.

FIG. 16 shows refractory periods for ventricular triggered biventricularpacing with inhibition of the left ventricular pace.

FIG. 17 shows refractory periods for ventricular triggered biventricularpacing where both ventricles are paced.

FIG. 18 shows refractory periods for ventricular triggered leftventricle-only pacing.

DESCRIPTION OF THE INVENTION

The present invention relates to a method and system for managingrefractory periods for multiple sensing channels in order to safely andeffectively implement a synchronized pacing mode. As will be describedbelow, such modes may be used to deliver cardiac resynchronizationtherapy.

1. Hardware Platform

Pacemakers are typically implanted subcutaneously or submuscularly andhave leads threaded intravenously into the heart to connect the deviceto electrodes used for sensing and pacing. A programmable electroniccontroller causes the pacing pulses to be output in response to lapsedtime intervals and sensed electrical activity (i.e., intrinsic heartbeats not as a result of a pacing pulse). Pacemakers sense intrinsiccardiac electrical activity by means of internal electrodes disposedwithin or near the chamber to be sensed. A depolarization waveassociated with an intrinsic contraction of the atria or ventricles thatis detected by the pacemaker is referred to as an atrial sense orventricular sense, respectively. In order to cause such a contraction inthe absence of an intrinsic beat, a pacing pulse (either an atrial paceor a ventricular pace) with energy above a certain pacing threshold isdelivered to the chamber.

FIG. 1 shows a system diagram of a microprocessor-based pacemakerphysically configured with sensing and pacing channels for both atriaand both ventricles. The controller 10 of the pacemaker is amicroprocessor which communicates with a memory 12 via a bidirectionaldata bus. The memory 12 typically comprises a ROM (read-only memory) forprogram storage and a RAM (random-access memory) for data storage. Thepacemaker has atrial sensing and pacing channels comprising electrode 34a-b, leads 33 a-b, sensing amplifiers 31 a-b, pulse generators 32 a-b,and atrial channel interfaces 30 a-b which communicate bidirectionallywith microprocessor 10. The device also has ventricular sensing andpacing channels for both ventricles comprising electrodes 24 a-b, leads23 a-b, sensing amplifiers 21 a-b, pulse generators 22 a-b, andventricular channel interfaces 20 a-b. In the figure, “a” designates oneventricular or atrial channel and “b” designates the channel for thecontralateral chamber. In this embodiment, a single electrode is usedfor sensing and pacing in each channel, known as a unipolar lead. Otherembodiments may employ bipolar leads which include two electrodes foroutputting a pacing pulse and/or sensing intrinsic activity. The channelinterfaces 20 a-b and 30 a-b include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters which can be written to by the microprocessor in order tooutput pacing pulses, change the pacing pulse amplitude, and adjust thegain and threshold values for the sensing amplifiers. An exertion levelsensor 330 (e.g., an accelerometer or a minute ventilation sensor)enables the controller to adapt the pacing rate in accordance withchanges in the patient's physical activity. A telemetry interface 40 isalso provided for communicating with an external programmer 500 whichhas an associated display 510. A pacemaker incorporating the presentinvention may possess all of the components in FIG. 1 and beprogrammable so as to operate in a number of different modes, or it mayhave only those components necessary to operate in a particular mode.

The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory. The controller10 controls the delivery of paces via the pacing channels, interpretssense signals from the sensing channels, implements timers for definingescape intervals and sensory refractory periods, and performs the pacecounting functions as described below. It should be appreciated,however, that these functions could also be performed by custom logiccircuitry either in addition to or instead of a programmedmicroprocessor.

2. Bradycardia Pacing Modes

Bradycardia pacing modes refer to pacing algorithms used to pace theatria and/or ventricles when the intrinsic ventricular rate isinadequate either due to AV conduction blocks or sinus node dysfunction.Such modes may either be single-chamber pacing, where either an atriumor a ventricle is paced, or dual-chamber pacing in which both an atriumand a ventricle are paced. The modes are generally designated by aletter code of three positions where each letter in the code refers to aspecific function of the pacemaker. The first letter refers to whichheart chambers are paced and which may be an A (for atrium), a V (forventricle), D (for both chambers), or O (for none). The second letterrefers to which chambers are sensed by the pacemaker's sensing channelsand uses the same letter designations as used for pacing. The thirdletter refers to the pacemaker's response to a sensed P wave from theatrium or an R wave from the ventricle and may be an I (for inhibited),T (for triggered), D (for dual in which both triggering and inhibitionare used), and O (for no response). Modern pacemakers are typicallyprogrammable so that they can operate in any mode which the physicalconfiguration of the device will allow. Additional sensing ofphysiological data allows some pacemakers to change the rate at whichthey pace the heart in accordance with some parameter correlated tometabolic demand. Such pacemakers are called rate-adaptive pacemakersand are designated by a fourth letter added to the three-letter code, R.

Pacemakers can enforce a minimum heart rate either asynchronously orsynchronously. In asynchronous pacing, the heart is paced at a fixedrate irrespective of intrinsic cardiac activity. There is thus a riskwith asynchronous pacing that a pacing pulse will be deliveredcoincident with an intrinsic beat and during the heart's vulnerableperiod which may cause fibrillation. Most pacemakers for treatingbradycardia today are therefore programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity. In an inhibited demand mode,a pacing pulse is delivered to a heart chamber during a cardiac cycleonly after expiration of a defined escape interval during which nointrinsic beat by the chamber is detected. If an intrinsic beat occursduring this interval, the heart is thus allowed to “escape” from pacingby the pacemaker. Such an escape interval can be defined for each pacedchamber. For example, a ventricular escape interval can be definedbetween ventricular events so as to be restarted with each ventricularsense or pace. The inverse of this escape interval is the minimum rateat which the pacemaker will allow the ventricles to beat, sometimesreferred to as the lower rate limit (LRL).

In atrial tracking pacemakers (i.e., VDD or DDD mode), anotherventricular escape interval is defined between atrial and ventricularevents, referred to as the atrio-ventricular interval (AVI). Theatrio-ventricular interval is triggered by an atrial sense or pace andstopped by a ventricular sense or pace. A ventricular pace is deliveredupon expiration of the atrio-ventricular interval if no ventricularsense occurs before. Atrial-tracking ventricular pacing attempts tomaintain the atrio-ventricular synchrony occurring with physiologicalbeats whereby atrial contractions augment diastolic filling of theventricles. If a patient has a physiologically normal atrial rhythm,atrial-tracking pacing also allows the ventricular pacing rate to beresponsive to the metabolic needs of the body.

A pacemaker can also be configured to pace the atria on an inhibiteddemand basis. An atrial escape interval is then defined as the maximumtime interval in which an atrial sense must be detected after aventricular sense or pace before an atrial pace will be delivered. Whenatrial inhibited demand pacing is combined with atrial-triggeredventricular demand pacing (i.e., DDD mode), the lower rate limitinterval is then the sum of the atrial escape interval and theatrio-ventricular interval.

Another type of synchronous pacing is atrial-triggered orventricular-triggered pacing. In this mode, an atrium or ventricle ispaced immediately after an intrinsic beat is detected in the respectivechamber. Triggered pacing of a heart chamber is normally combined withinhibited demand pacing so that a pace is also delivered upon expirationof an escape interval in which no intrinsic beat occurs. Such triggeredpacing may be employed as a safer alternative to asynchronous pacingwhen, due to far-field sensing of electromagnetic interference fromexternal sources or skeletal muscle, false inhibition of pacing pulsesis a problem. If a sense in the chamber's sensing channel is an actualdepolarization and not a far-field sense, the triggered pace isdelivered during the chamber's physiological refractory period and is ofno consequence.

3. Cardiac Resynchronization Therapy

Cardiac resynchronization therapy is pacing stimulation applied to oneor more heart chambers in a manner that restores or maintainssynchronized bilateral contractions of the atria and/or ventricles andthereby improves pumping efficiency. Certain patients with conductionabnormalities may experience improved cardiac synchronization withconventional single-chamber or dual-chamber pacing as described above.For example, a patient with left bundle branch block may have a morecoordinated contraction of the ventricles with a pace than as a resultof an intrinsic contraction. In that sense, conventional bradycardiapacing of an atrium and/or a ventricle may be considered asresynchronization therapy. Resynchronization pacing, however, may alsoinvolve pacing both ventricles and/or both atria in accordance with asynchronized pacing mode as described below. A single chamber may alsobe resynchronized to compensate for intra-atrial or intra-ventricularconduction delays by delivering paces to multiple sites of the chamber.

It is advantageous to deliver resynchronization therapy in conjunctionwith one or more synchronous bradycardia pacing modes, such as aredescribed above. One atrial and/or one ventricular pacing sites aredesignated as rate sites, and paces are delivered to the rate sitesbased upon pacing and sensed intrinsic activity at the site inaccordance with the bradycardia pacing mode. In a single-chamberbradycardia pacing mode, for example, one of the paired atria or one ofthe ventricles is designated as the rate chamber. In a dual-chamberbradycardia pacing mode, either the right or left atrium is selected asthe atrial rate chamber and either the right or left ventricle isselected as the ventricular rate chamber. The heart rate and the escapeintervals for the pacing mode are defined by intervals between sensedand paced events in the rate chambers only. Resynchronization therapymay then be implemented by adding synchronized pacing to the bradycardiapacing mode where paces are delivered to one or more synchronized pacingsites in a defined time relation to one or more selected sensing andpacing events that either reset escape intervals or trigger paces in thebradycardia pacing mode. Multiple synchronized sites may be pacedthrough multiple synchronized sensing/pacing channels, and the multiplesynchronized sites may be in the same or different chambers as the ratesite.

In bilateral synchronized pacing, which may be either biatrial orbiventricular synchronized pacing, the heart chamber contralateral tothe rate chamber is designated as a synchronized chamber. For example,the right ventricle may be designated as the rate ventricle and the leftventricle designated as the synchronized ventricle, and the paired atriamay be similarly designated. Each synchronized chamber is then paced ina timed relation to a pace or sense occurring in the contralateral ratechamber.

One synchronized pacing mode may be termed offset synchronized pacing.In this mode, the synchronized chamber is paced with a positive,negative, or zero timing offset as measured from a pace delivered to itspaired rate chamber, referred to as the synchronized chamber offsetinterval. The offset interval may be zero in order to pace both chamberssimultaneously, positive in order to pace the synchronized chamber afterthe rate chamber, or negative to pace the synchronized chamber beforethe rate chamber. One example of such pacing is biventricular offsetsynchronized pacing where both ventricles are paced with a specifiedoffset interval. The rate ventricle is paced in accordance with asynchronous bradycardia mode which may include atrial tracking, and theventricular escape interval is reset with either a pace or a sense inthe rate ventricle. (Resetting in this context refers to restarting theinterval in the case of an LRL ventricular escape interval and tostopping the interval in the case of an AVI.) Thus, a pair ofventricular paces is delivered after expiration of the AVI escapeinterval or expiration of the LRL escape interval, with ventricularpacing inhibited by a sense in the rate ventricle that restarts the LRLescape interval and stops the AVI escape interval. In this mode, thepumping efficiency of the heart will be increased in some patients bysimultaneous pacing of the ventricles with an offset of zero. However,it may be desirable in certain patients to pace one ventricle before theother in order to compensate for different conduction velocities in thetwo ventricles, and this may be accomplished by specifying a particularpositive or negative ventricular offset interval.

Another synchronized mode is triggered synchronized pacing. In one typeof triggered synchronized pacing, the synchronized chamber is pacedafter a specified trigger interval following a sense in the ratechamber, while in another type the rate chamber is paced after aspecified trigger interval following a sense in the synchronizedchamber. The two types may also be employed simultaneously. For example,with a trigger interval of zero, pacing of one chamber is triggered tooccur in the shortest time possible after a sense in the other chamberin order to produce a coordinated contraction. (The shortest possibletime for the triggered pace is limited by a sense-to-pace latency perioddictated by the hardware.) This mode of pacing may be desirable when theintra-chamber conduction time is long enough that the pacemaker is ableto reliably insert a pace before depolarization from one chamber reachesthe other. Triggered synchronized pacing can also be combined withoffset synchronized pacing such that both chambers are paced with thespecified offset interval if no intrinsic activity is sensed in the ratechamber and a pace to the rate chamber is not otherwise delivered as aresult of a triggering event. A specific example of this mode isventricular triggered synchronized pacing where the rate andsynchronized chambers are the right and left ventricles, respectively,and a sense in the right ventricle triggers a pace to the left ventricleand/or a sense in the left ventricle triggers a pace to the rightventricle.

As with other synchronized pacing modes, the rate chamber in a triggeredsynchronized pacing mode can be paced with one or more synchronousbradycardia pacing modes. If the rate chamber is controlled by atriggered bradycardia mode, a sense in the rate chamber sensing channel,in addition to triggering a pace to the synchronized chamber, alsotriggers an immediate rate chamber pace and resets any rate chamberescape interval. The advantage of this modal combination is that thesensed event in the rate chamber sensing channel might actually be afar-field sense from the synchronized chamber, in which case the ratechamber pace should not be inhibited. In a specific example, the rightand left ventricles are the rate and synchronized chambers,respectively, and a sense in the right ventricle triggers a pace to theleft ventricle. If right ventricular triggered pacing is also employedas a bradycardia mode, both ventricles are paced after a rightventricular sense has been received to allow for the possibility thatthe right ventricular sense was actually a far-field sense of leftventricular depolarization in the right ventricular channel. If theright ventricular sense were actually from the right ventricle, theright ventricular pace would occur during the right ventricle'sphysiological refractory period and cause no harm.

As mentioned above, certain patients may experience some cardiacresynchronization from the pacing of only one ventricle and/or oneatrium with a conventional bradycardia pacing mode. It may be desirable,however, to pace a single atrium or ventricle in accordance with apacing mode based upon senses from the contralateral chamber. This mode,termed synchronized chamber-only pacing, involves pacing only thesynchronized chamber based upon senses from the rate chamber. One way toimplement synchronized chamber-only pacing is to pseudo-pace the ratechamber whenever the synchronized chamber is paced before the ratechamber is paced, such that the pseudo-pace inhibits a rate chamber paceand resets any rate chamber escape intervals. Such pseudo-pacing can becombined with the offset synchronized pacing mode using a negativeoffset to pace the synchronized chamber before the rate chamber and thuspseudo-pace the rate chamber, which inhibits the real scheduled ratechamber pace and resets the rate chamber pacing escape intervals. Oneadvantage of this combination is that sensed events in the rate chamberwill inhibit the synchronized chamber-only pacing, which may benefitsome patients by preventing pacing that competes with intrinsicactivation (i.e., fusion beats). Another advantage of this combinationis that rate chamber pacing can provide backup pacing when in asynchronized chamber-only pacing mode, such that when the synchronizedchamber pace is prevented, for example to avoid pacing during thechamber vulnerable period following a prior contraction, the ratechamber will not be pseudo-paced and thus will be paced upon expirationof the rate chamber escape interval. Synchronized chamber-only pacingcan be combined also with a triggered synchronized pacing mode, inparticular with the type in which the synchronized chamber is triggeredby a sense in the rate chamber. One advantage of this combination isthat sensed events in the rate chamber will trigger the synchronizedchamber-only pacing, which may benefit some patients by synchronizingthe paced chamber contractions with premature contralateral intrinsiccontractions.

An example of synchronized chamber-only pacing is left ventricle-onlysynchronized pacing where the rate and synchronized chambers are theright and left ventricles, respectively. Left ventricle-onlysynchronized pacing may be advantageous where the conduction velocitieswithin the ventricles are such that pacing only the left ventricleresults in a more coordinated contraction by the ventricles than withconventional right ventricular pacing or biventricular pacing. Leftventricle-only synchronized pacing may be implemented in inhibiteddemand modes with or without atrial tracking, similar to biventricularpacing. A left ventricular pace is then delivered upon expiration of theAVI escape interval or expiration of the LRL escape interval, with leftventricular pacing inhibited by a right ventricular sense that restartsthe LRL escape interval and stops the AVI escape interval.

In the synchronized modes described above, the rate chamber issynchronously paced with a mode based upon detected intrinsic activityin the rate chamber, thus protecting the rate chamber against pacesbeing delivered during the vulnerable period. In order to providesimilar protection to a synchronized chamber or synchronized pacingsite, a synchronized chamber protection period (SCPP) may be provided.(In the case of multi-site synchronized pacing, a similar synchronizedsite protection period may be provided for each synchronized site.) TheSCPP is a programmed interval which is initiated by sense or paceoccurring in the synchronized chamber during which paces to thesynchronized chamber are inhibited. For example, if the right ventricleis the rate chamber and the left ventricle is the synchronized chamber,a left ventricular protection period LVPP is triggered by a leftventricular sense which inhibits a left ventricular pace which wouldotherwise occur before the escape interval expires. The SCPP may beadjusted dynamically as a function of heart rate and may be differentdepending upon whether it was initiated by a sense or a pace. The SCPPprovides a means to inhibit pacing of the synchronized chamber when apace might be delivered during the vulnerable period or when it mightcompromise pumping efficiency by pacing the chamber too close to anintrinsic beat. In the case of a triggered mode where a synchronizedchamber sense triggers a pace to the synchronized chamber, the pacingmode may be programmed to ignore the SCPP during the triggered pace.Alternatively, the mode may be programmed such that the SCPP starts onlyafter a specified delay from the triggering event, which allowstriggered pacing but prevents pacing during the vulnerable period.

In the case of synchronized chamber-only synchronized pacing, asynchronized chamber pace may be inhibited if a synchronized chambersense occurs within a protection period prior to expiration of the ratechamber escape interval. Since the synchronized chamber pace isinhibited by the protection period, the rate chamber is not pseudo-pacedand, if no intrinsic activity is sensed in the rate chamber, it will bepaced upon expiration of the rate chamber escape intervals. The ratechamber pace in this situation may thus be termed a safety pace. Forexample, in left ventricle-only synchronized pacing, a right ventricularsafety pace is delivered if the left ventricular pace is inhibited bythe left ventricular protection period and no right ventricular sensehas occurred.

As noted above, synchronized pacing may be applied to multiple sites inthe same or different chambers. The synchronized pacing modes describedabove may be implemented in a multi-site configuration by designatingone sensing/pacing channel as the rate channel for sensing/pacing a ratesite, and designating the other sensing/pacing channels in either thesame or the contralateral chamber as synchronized channels forsensing/pacing one or more synchronized sites. Pacing and sensing in therate channel then follows rate chamber timing rules, while pacing andsensing in the synchronized channels follows synchronized chamber timingrules as described above. The same or different synchronized pacingmodes may be used in each synchronized channel.

4. Refractory Period Management

The present invention is a system and method for managing refractoryperiods. A refractory period is an interval during which sensed activityfrom a sensing channel neither inhibits nor triggers a pacing pulse.However, sensed events during the refractory period are often used forother device algorithms such as those used to detect atrial arrhythmiasand those used to extend the refractory period in the presence ofpersistent electromagnetic interference.

Refractory periods are often implemented using a blanking interval. Whenused, the blanking interval usually constitutes the first part of therefractory period. During the blanking interval, the device ignores allelectrical activity sensed by the sensing channel. Blanking intervalscan shield the sensing channel from pacing artifacts or cross-chamberdepolarization effects. Refractory periods are also often implementedusing a retriggerable noise interval, where the noise interval oftenconstitutes the last part of the refractory period. Sensed eventsoccurring within the noise interval will restart the noise interval,thus increasing the length of the refractory period.

The following description of the invention dealing with refractoryperiod management is set forth with respect to an exemplary cardiacrhythm management device that provides ventricular pacing in asynchronized mode and in which the rate and synchronized chambers arethe right and left ventricles, respectively. It should be appreciated,however, that similar embodiments could be constructed in accordancewith the description by replacing the right and left ventricles withother heart chambers or cardiac sites designated as rate andsynchronized chambers.

The exemplary device provides sensing/pacing channels for an atrium andboth ventricles. FIGS. 2-18 depict cardiac events and illustrate therefractory periods for the sensing channels. Such refractory periods maybe provided by timers implemented in hardware or software executed bythe controller in FIG. 1. Each refractory period is defined by itssensing channel and the particular cardiac or pacemaker event thattriggers it. Refractory periods following sensed (i.e., intrinsic) orpaced events will be referred to as sensing or pacing refractoryperiods, respectively.

FIG. 2 shows the refractory periods of the atrial and both right andleft ventricular channels after a sensed atrial event. The atrialchannel is rendered refractory for a selected interval ARPAS whichprevents any electrical events occurring after the atrial depolarizationand before expiration of the interval from being interpreted as anatrial sense. The right ventricular channel continues normal sensing aswith conventional pacemakers since far-field sensing of the atrium by aright ventricular electrode is unlikely. A refractory period for theleft ventricular channel may be necessary, however. Since a leftventricular electrode is normally placed via the coronary sinus, it maybe placed rather high in ventricle in close proximity to the leftatrium. In order to prevent oversensing of the left atrium by the leftventricular channel, the channel can be made refractory for a selectedinterval LVRPAS.

FIG. 3 depicts the refractory periods following an atrial pace. Theatrial, right ventricular, and left ventricular channels are renderedrefractory for selected intervals ARPAP, RVRPAP, and LVRPAP,respectively. Note that although RVRPAP and LVRPAP are shown as beingequal, it may be advantageous to extend the left ventricular refractoryperiod beyond the right ventricular refractory period in order to allowfor the added time that a depolarization from a right atrial pace takesto reach the left atrium where it could possibly be oversensed by theleft ventricular channel. There may also be instances where it would beadvantageous to set the RVRPAP longer than the LVRPAP.

FIG. 4 shows the sensing refractory periods after a sensed intrinsicright ventricular depolarization. The atrial and right ventricularchannels are rendered refractory for separately selected intervalsARPRVS (which is commonly referred to as the post-ventricular atrialrefractory period or PVARP, used primarily to prevent pacemaker mediatedtachycardia) and RVRPRVS as is done for a conventional dual-chamberpacemaker with sensing only in the right ventricle. The left ventricularsensing channel remains non-refractory, however, in order to continuesensing for a depolarization which would inhibit a left ventricular paceoutput and to allow diagnostic functions in the cardiac rhythmmanagement device to count the intrinsic left ventricular event. Forleft ventricular pacing in which the lower rate limit interval iscontrolled by right ventricular events (i.e., where pacing of eitherventricle is inhibited by a right ventricular sense), it is desirable toprovide a time interval triggered by a left ventricular sense signalduring which pacing of the left ventricle is inhibited. That inhibitoryinterval, referred to herein as LVPP for left ventricular protectiveperiod, serves to prevent a left ventricular pace during the vulnerableperiod following a depolarization which in some patients can triggerventricular arrhythmias.

FIG. 5 shows the refractory periods of the sensing channels following anintrinsic left ventricular event. Only the left ventricle channel isrendered refractory for a selected interval LVRPLVS. In order not toimpact pacing algorithms dependent upon right ventricular and atrialevents, it is advantageous to not initiate refractory periods in eitherof these sensing channels. Note that an LVPP interval may also betriggered by the left ventricular sense.

FIGS. 6 through 10 show the refractory periods of the sensing channelsafter ventricular pace events when the pacemaker is operated inbiventricular pace mode with no ventricular triggering. In this mode,ventricular pacing is triggered by an atrial sense after expiration ofthe AVI or expiration of the LRL interval. In either case, ventricularpacing is inhibited by a right ventricular sense. A left ventricularpace is output offset from a right ventricle pace by a biventricularoffset interval BVD (i.e., a synchronized chamber offset interval). Theinterval BVD may be zero in order to pace both ventriclessimultaneously, positive in order to pace the left ventricle after theright, or negative if the left ventricle is paced before the right.

FIG. 6 shows the pacing refractory periods defined by intervals ARPVP,RVRPVP, and LVRPVP for the atrial, right ventricular, and leftventricular channels, respectively, after a biventricular pace with apositive BVD. Intervals ARPVP, RVRPVP and LVRPVP start at the instant ofthe right ventricular pace. It should be noted that, in order to preventsensing the depolarization produced by the pace, the left ventricularchannel should be rendered refractory following a right ventricle pacefor a period of time long enough to encompass the time it takes for adepolarization wave to reach the left ventricle. The right to leftventricle conduction time may be short enough so that no additionalrefractory time is needed following the left ventricular pace whichoccurs after the delay interval BVD. However, it may be advantageous tomaintain a minimum interval MINREF from the left ventricular pace to theend of the RVRPVP and LVRPVP. This means that pacing refractoryintervals for the atrial and both ventricular sensing channels inbiventricular pacing mode can be defined so as to be triggered bywhichever ventricular pace occurs first. Thus both RVRPVP and LVRPVP canbe triggered by the right ventricular pace when BVD is positive. FIG. 8shows the refractory periods for biventricular pacing mode in which theleft ventricle is paced first followed by a right ventricular pace aftera selected biventricular delay interval BVD (i.e., BVD is negative). Therefractory period intervals ARPVP, RVRPVP, and LVRPVP for the atrial andventricular channels are similar to those of

FIG. 6 except that they are triggered by a left ventricular pace. FIG.10 shows the refractory periods in biventricular pacing mode with theventricles paced simultaneously (i.e., when BVD is zero) again showingthe intervals ARPVP, RVRPVP, and LVRPVP triggered by either of theventricular paces.

In another embodiment, during biventricular pacing with a non-zero BVD,the refractory periods for the atrium and ventricles are started on thelast ventricular pace. This is depicted in FIGS. 11 and 12. As shown,refractory periods ARPVP1, RVRPVP1, and LVRPVP1 are triggered by theleading ventricular pace. The lagging ventricular pace triggersrefractory periods ARPVP2, RVRPVP2, and LVRPVP2. If any of the ARPVP1,RVRPVP1, or LVRPVP1 periods exceed the BVD, that refractory period istruncated at the time of the lagging ventricular pace. The ARPVP1,RVRPVP1, and LVRPVP1 periods may be the same or different as the ARPVP2,RVRPVP2, and LVRPVP2 periods.

In any of the non-ventricular-triggered biventricular pacing modes, theleft ventricular pace is inhibited if the pace would fall within theleft ventricular protective period interval LVPP. The LVPP may betriggered by a left ventricular sense occurring before the rightventricular pace (i.e., before the onset of the left ventricularrefractory period). In that event, as shown in FIG. 7 with respect to abiventricular pacing mode with a positive BVD, the refractory intervalsare triggered by the right ventricular pace and are left unchanged fromthose of FIG. 6. The event labeled as a left ventricular pseudo-pacerepresents where the left ventricular pace would have occurred were itnot inhibited. FIG. 9 shows that the refractory intervals are triggeredby the right ventricular pace when the right ventricular pace is tofollow the left ventricular pace (i.e., BVD is negative) but no leftventricular pace occurs due to triggering of the LVPP. The pacingrefractory periods with simultaneous ventricular pacing are similarlyunchanged when left ventricular pacing is inhibited. Thus the earliestventricular pace output occurring can be used to trigger the pacingrefractory periods regardless of the value of the biventricular delayand whether or not the left ventricular protective period is triggered.In another embodiment, the refractory periods are triggered by the rightventricular pace but set to a value different than that used duringbiventricular pacing.

In some situations, the best hemodynamic effect may result from pacingonly the right or left ventricle. FIGS. 13 and 14 depict the refractoryperiods associated with right-only and left-only ventricular pacing,respectively. During left-only ventricular pacing, the left ventricularpaces are inhibited by either left ventricular or right ventricularsensed events. FIG. 15 depicts the refractory periods after a rightventricular safety pace delivered during left ventricle-only pacing whenthe left ventricle pace is inhibited by an LVPP. During right-onlyventricular pacing, right ventricular paces are inhibited by only rightventricular sensed events. Since left ventricular sensing may still beused for purposes other than pace inhibition, such as diagnosticcounting of left ventricular senses, implementation of a refractoryperiod for the left ventricular sensing channel may be necessary whilein a right-only ventricular pacing mode. As shown in FIGS. 13, 14, and15, all refractory periods ARPVP, RVRPVP, and LVRPVP start at theinstant of the right ventricular pace in the case of right-onlyventricular pacing, and at the instant of the left ventricular pace inthe case of left-only ventricular pacing.

FIGS. 16, 17, and 18 show the refractory intervals of the sensingchannels when the pacemaker is operated in biventricular pacing modewith ventricular triggering. In this mode, rather than inhibiting pacingupon receipt of a right ventricular sense, ventricular pacing istriggered to occur in the shortest time possible after the sense inorder to produce a coordinated contraction of the ventricles. This modeof pacing may be desirable when the inherent intraventricular conductiontime of the heart is long enough that the pacemaker is able to reliablyinsert a pace before depolarization from the right ventricle would reachthe left ventricle. The time delay between a right ventricular sense andthe ensuing pace output is dictated by the response time of the hardwareand is designated in FIGS. 16, 17, and 18 by the sense to pace latencyinterval SPL. The mode may operate such that following a rightventricular sense, either the left ventricle only is paced as shown inFIGS. 16 and 18, or both ventricles are paced as shown in FIG. 17. Inthe latter case, the right ventricle is paced even though a rightventricular sense has been received to allow for the possibility thatthe right ventricular sense was actually a far-field left ventricularsense in the right ventricular channel. If the right ventricular sensewere actually from the right ventricle, the right ventricular pace wouldoccur during the right ventricle's physiological refractory period andbe of no consequence. With either type of ventricular triggeredbiventricular pacing mode, pacing of the left ventricle is inhibited ifit would be delivered during the LVPP interval. FIGS. 16, 17, and 18show that in ventricular triggered mode, separately selectablerefractory period intervals ARPVT, RVRPVT, and LVRPVT for the atrial andventricular sensing channels are triggered by either a univentricular orbiventricular pace. FIGS. 16, 17, and 18 also show that the rightventricle and atrium are refractory during the SPL. The atrial and rightventricular refractory periods during the SPL (ARPRVS, RVRPRVS) aretriggered by the right ventricular sense and are truncated at the timeof the ventricular pace.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A cardiac rhythm management device, comprising:sensing and pacing circuitry connectable to a right ventricularelectrode and a left ventricular electrode to form right and leftventricular pacing and sensing channels; a controller connected to thesensing/pacing circuitry and programmed to: generate a right ventricularsense upon detection of depolarization above a specified thresholdoccurring in the right ventricle; generate a left ventricular sense upondetection of depolarization above a specified threshold occurring in theleft ventricle; pace the left ventricle by delivering a pacing pulse tothe left ventricular electrode upon expiration of an escape interval;restart the escape interval after a right ventricular sense; render theright and left ventricular sensing channels refractory for a selectedpacing refractory period after delivery of a left ventricular pace; andrender the right ventricular sensing channel refractory for a selectedsensing refractory period after generation of a right ventricular sensewhile leaving the left ventricular sensing channel non-refractory. 2.The device of claim 1 wherein the controller is further programmed torestart the escape interval after a left ventricular pace.
 3. The deviceof claim 1 wherein the controller is further programmed to pace the leftand right ventricles in accordance with expiration of the escapeinterval, wherein the left ventricle is paced before the rightventricle.
 4. The device of claim 3 wherein the controller is furtherprogrammed to pace the right ventricle upon expiration of the escapeinterval and pace the left ventricle at a pacing instant defined tooccur prior to expiration of the escape interval by a specified offsetinterval.
 5. The device of claim 4 wherein the controller is furtherprogrammed to restart the escape interval after a right ventricularpace.
 6. A cardiac rhythm management device, comprising: sensing andpacing circuitry connectable to an atrial electrode to form an atrialsensing and pacing channels and connectable to a right ventricularelectrode and a left ventricular electrode to form right and leftventricular sensing and pacing channels; a controller connected to thesensing/pacing circuitry and programmed to: generate a right ventricularsense upon detection of depolarization above a specified thresholdoccurring in the right ventricle; generate a left ventricular sense upondetection of depolarization above a specified threshold occurring in theleft ventricle; generate an atrial sense upon detection an atrialdepolarization above a specified threshold; initiate anatrio-ventricular escape interval after an atrial event; pace the leftventricle by delivering a pacing pulse to the left ventricular electrodeupon expiration of the atrio-ventricular escape interval; stop theatrio-ventricular escape interval after a right ventricular sense;render the right and left ventricular sensing channels refractory forselected pacing refractory periods after delivery of left ventricularpace; and render the ventricular sensing channel refractory for aselected sensing refractory period after generation of a rightventricular sense while leaving the left ventricular sensing channelnon-refractory.
 7. The device of claim 6 wherein the controller isfurther programmed to pace the left and right ventricles in accordancewith expiration of the atrio-ventricular escape interval, wherein theleft ventricle is paced before the right ventricle.
 8. The device ofclaim 6 wherein the controller is further programmed to: start theatrio-ventricular escape interval upon occurrence of an atrial sense;render the atrial sensing channel refractory for a selected pacingrefractory period after a ventricular pacing event; and, render theatrial sensing channel refractory for a selected sensing refractoryperiod after a ventricular sense.
 9. The device of claim 6 wherein thecontroller is further programmed to pace the right ventricle uponexpiration of the atrio-ventricular escape interval and pace the leftventricle at a pacing instant defined to occur prior to expiration ofthe atrio-ventricular escape interval by a specified offset interval.10. The device of claim 6 wherein the controller is further programmedto: pace an atrium upon expiration of an atrial escape interval which isrestarted by an atrial pace or an atrial sense; and, start theatrio-ventricular escape interval after an atrial pace.
 11. A cardiacrhythm management device, comprising: sensing circuitry connectable to afirst electrode for forming a first ventricular sensing channel and asecond electrode for forming a second ventricular sensing channel;pacing circuitry connectable to the second electrode to form a pacingchannel for delivering paces to the second ventricle; a controllerconnected to the sensing and pacing circuitry and programmed to generatea first ventricular sense signal upon detection of depolarizationoccurring in the first ventricle and a second ventricular sense signalupon detection of depolarization occurring in the contralateral secondventricle; wherein the controller is further programmed to: pace thesecond ventricle within a specified trigger interval after generation ofa first ventricular sense signal; render each sensing channel refractoryfor a separately selected pacing refractory period after delivery of apace; and, render the first ventricular sensing channel refractory for aselected sensing refractory period after generation of a firstventricular sense signal.
 12. The device of claim 11 wherein thecontroller is further programmed to pace the first ventricle within aspecified trigger interval after generation of a second ventricle sensesignal.
 13. The device of claim 11 wherein the controller is furtherprogrammed to pace the first ventricle, designated as a rate chamber,upon expiration of a rate chamber escape interval which is reset by arate chamber sense signal.
 14. The device of claim 13 wherein thecontroller is further programmed to pace the second ventricle,designated as a synchronized chamber, at a specified synchronizedchamber offset interval measured with respect to a rate chamber pace.15. The device of claim 14 wherein the specified synchronized chamberoffset interval is zero so that both ventricles are paced simultaneouslyupon expiration of the rate chamber escape interval.
 16. The device ofclaim 14 wherein the specified synchronized chamber offset interval ispositive so that the synchronized chamber is paced after a pace isdelivered to the rate chamber upon expiration of the rate chamber escapeinterval.
 17. The device of claim 11 wherein the first ventricle isdesignated as a rate chamber and the second ventricle is designated as asynchronized chamber and wherein the controller is further programmed topace the synchronized chamber upon expiration of a rate chamber escapeinterval which is reset by a rate chamber sense signal.
 18. The deviceof claim 11 wherein the controller is further programmed to pace thefirst ventricle immediately after the generation of a first ventricularsense signal.
 19. The device of claim 13 wherein the controller isfurther programmed to pace the rate chamber and resetting the ratechamber escape interval immediately after generation of a rate chambersense signal.
 20. The device of claim 1 wherein the controller isfurther programmed to pace the first ventricle upon expiration of anatrio-ventricular escape interval which is started by an atrial sensesignal and stopped by a first ventricle sense signal.